Oxygen performance of contact lenses: A human eye model

Since Smesler and Ozanics1 demonstrated the affinity of the cornea for atmospheric oxygen, contact lens investigators have become increasingly concerned with maximising oxygen availability to the contact lens wearing cornea. As a result, many methods have been adopted to evaluate the oxygen delivery characteristics of different contact lens materials and designs. Fatt and St Helen2 described a modified electrode chamber technique that facilitated the measurement of oxygen transmissibility of hydrogel lenses. The results are expressed as DK/L values, where D is the co-efficient of diffusion for oxygen movement in the lens material, K is the solubility of oxygen in that material and L is the lens centre thickness. This method has been used extensively. For example, Morrison and Edelhauser3, and later Morris and Fatt4 determined the oxygen permeability of a number of commercially available contact lenses, while Wilson et al5 in a more clinical approach, compared a range of standard lenses of fixed back vertex power. Wilson6 has also used this technique to examine the effect of lens power on the relationship between centre thickness and oxygen transmissibility. Mathematical models, based on these oxygen transmissibility measurements and oxygen consumption rate studies of rabbit cornea, were developed by Fatt et al7-9 to predict ranges of oxygen tensions at the cornea-contact lens interface for various contact lens materials. Hill10 has described an equivalent oxygen technique that provides, on the other hand an invivo estimation of contact lens oxygen performance in the rabbit cornea.11 The technique involves comparing corneal oxygen uptake after contact lens wear with that after exposure to known oxygen environments, and the results are expressed as an equivalent oxygen percentage (EOP). This method has been used extensively to assess various aspects of physiology of contact lens wear.12-15 A third approach to the assessment of oxygen availability during contact lens wear is that of Polse and Decker.16 They compared human corneal swelling responses after contact lens wear to that after corneal exposure to known oxygen environments. How well then, do these three approaches predict the level of oxygen beneath contact lenses during normal wear? In this study, a recently described technique17 is used to determine the EOPs beneath a variety of contact lens types in the in vivo human eye during normal wearing conditions.